Sweep protection circuit



Jan. 5, 1965 G. E. BRooMHALl.

swEEP PROTECTION CIRCUIT 2 Sheets-Sheet 1 Filed July 2, 1962 M FCM? Jan. 5, 1965 G. E. BROOMHALL 3,164,746

swEEP PROTECTION CIRCUIT Filed July 2, 1962 2 Sheets-Sheet 2 IP f f V/ d l -2001/ -E/Z' .az/zag Wi/waa uw RM United States PatentOfi 3,154,746 SWE-EP PROTECTHN CIECUH Gordon` E. Broonahali, Escondido, Caiit., assigner to Hughes Aircrt Company, Culver City, Calif., a corporation of Deiaware Filed .uly 2, 11.591629 Ser. No. 296377 p 3 claims. (ci. sis- 2nA y v This invention relates to improved protection circuitsv for use with electron discharge devices such as a cathode ray tube and in particular to an improved circuit means for preventing damage to the sensitive luminescent screen of the discharge device upon reduction in potential of the energizing source for the electron beam deflection members. v

Cathode ray tube protection circuits have'been employed to prevent damage to the phosphor of. television kinescopes, or Oscilloscopes when either the horizontal slow sweep or the vertical fast' sweep voltages fail. The

failure of either continuous sweep signal causes the eie'c-V tron scanning beam to be deflected along one axis producing a single line on the luminescent screen. If the beam current is sufliciently large and if a suiicientl'y high accelerating voltage is applied to the anode of the cathode ray tube this line will be burned into the screen phosphor and will produce a blemish. This problem lis particularly acute during the aging process of the phosphor surface of the viewing screen. To properly age the phosphor surface it is continuously scanned for many hoursV by an electron beam of uniform intensity. Because of the necessity that` this process be carried on over a continuous time interval,

often longer than 24' hours, it is necessary to provide in the scanning circuit a means todetect automaticallythe loss of the sweep voltage. Y

The necessity for automatically and rapidly detecting the loss of the fast sweep voltage is not as critical as de- 'below a predetermined value.

f In'general, in its preferred form, the -present invention Acomprises in combination with a cathode raytube including a-high voltage circuit and-aslow sweep voltage generator a protection circuit comprising meansto obtain a signal representative of the maximum Aamplitude of the slow sweep voltage and means to disable the -high Vvoltage circuit of the cathode ray `tube when'said signal v`drops belowapredeterminedlevel. v n 1 Other advantages of the invention will hereinafter bei muni amplitude of the slow sweep deliectfionyoltage drops i Y come more fully apparent fromthefollowing description whereiny v FIGURE 1 is a schematic ,circuit'diagrami and a block lof the voltage output of theiclipping circuit. l

of the drawingswliich illustrate a preferred embodiment,

-serieszrela-tion between the lpositive terminal of .aisweep .voltage generator 11 and apoint of xedr'eference p0'- tentialsuch asggr'ound. The sweepvoltage generator 11 maybe arc'onvent-ion'al slow sweep s'awtoofth 'voltage' generator, Ysuch-as a Model `162 1nanufactfure'd'b y.A Tektronix, lne., havingga typicaloutput vvoltage waveformv as shown in FIG. 2. 'The output of the differentiating circuiti() isV applied to the control grid 2 3:y ofgarrst triode electron discharge device 21 forming partl of a voltage amplier K and comparator circuiti 20 villustrated here as one-half of tecting'the loss of `the slow sweep voltage because the fast sweep voltage causes the electron beam to move at such a speed thatla decrease in this voltage will not as readily causei a blemish on the phosphor screen. One .prior art method to detect-the losspof the Yfast Vsweep .voltage is to use a rectification and iiltering circuit to developa D.C. voltage proportional to sweep amplitude to operate a con trol device or circuit. However, such a circuit ycannot be effectively utilized to detect the loss of the horizontal speed voltage of the order of one sweep per second because it` requires a large filter of from l0 to 20 seconds time' conv stant which would inherently have a very slow response to a loss in sweep or a reduction'in sweep amplitude.

Another prior method used to prevent damageY to the senstiv'e luminescent' screen of a cathode rayI tube senses any change in potentialof the energizing source for the electron beam deflection circuit and uses this information to activate a protective circuit. While this method does protect against changes in the potential of the energizing source for the sweep voltage generator it does against changes in the sweep voltage itself.

`not protect v farad'iand `the .resistorV 15 avalue of 'l megohrn, and'in .a -dualitriode which isftypicallyof al2AX7 type. Typical#V Y it-he value of the capacitor 12 andresistor 15 are chosen toy provide ia relatively' shorttirne constantV RC'COllPlingV network compared to the 4sweep time of the orderf-of secondwherei-rr the capacitor 12 hase-airvalueof 0,2.v microaddition toprovider-as shown in FIG. 3f,fin-earch cycle-of the diierentiated waveform, -alirst portion A of` negative polarity-typically of a magnitudehof approximatelyy 200 l ,voltssrelative to ground andv a second portion YB of a; posi tive. polarity: typicallyr ofl a magnitude substantially less than. the said i relative to ground.

The anedes 24,729 Ofeaehef the dirai-.tueries ai; 22,

respectiyely, are supplied with .anoperatingjpotential from a terminal i6 through aload resistor S0 and thecatlzlode Therefore, one object of the present invention is to proi vide means for automatically and rapidly protecting the viewing screen of an electron dischargedevice` when the maximum amplitude ofthe horizontal sweepjdeliection volta-ge drops below a predetermined level. y'

A further object of the present invention is'to-fprovide an improved protection circuit forcathode ray tubes to prevent blemishes onthe highly sensitive phosphor screen by disabling the high voltage anode supply when the maximum amplitude ofthe slowsweep deflection voltages drops below a predetermined level. v s.v

A still further object of the present invention is to provide an; improved protectionY circuit for cathode .ray tubes to prevent blemish or damage to the phosphor screenof the tube during the yaging process thereof when themaxithe'dierentiating circuit .1Q and ground'.-

of eachisxsupplied with a negative potentialfwith' respect to: ground from a .terminal 13 4through Aarcornmon resistor 24. The source of potential may bea 'battery orgpower supply coupled with'thejterminals 13 and ,156 and ground to provideproper voltages. KV'Thecontrol grid25jV ofthe second'ltriode 422 ot-the dual triodek'is coupledfto a variable resistorv .27e suchas a conventional potentiometer for Y sensitivity control of the point of operation ofthe-pro-l tection circuit. T he potentiometer 261ispart of a voltage dividing network connected between the. positive'terminal 116 and-.ground includingv .asV well as potentiolneter26,A

resistors 27. 211x123..l ,j

As shown iniFIGSKQ. 1. and 4 `onlyithe postivefportion' of the--dilierentialsignalV is applied `to the control grid 23 since :the y negative first` portion` Awof Vthe diterentilated waveform isheld at 'groun'df-potential'rby y.rneansof ashunnt-V z; ing or kclamping circuit 14 includinganfelectron; discharge deviceflwhich is typically aldiode, such as a v61.3lL5gtype;`r having-an anode i9 andia Ycathode 18 is coupled between .i

3,164,746; Y' "'PatentekdJam 5, 1965 jrstY portion A andv in the order of '5 :volts In normal operation the second triode 22 is biased below its cutoff point through the adjustment of the potentiometer 26 and the first triode 21 is biased to render it conductive through the time interval of the second portion B of the (differentiated waveform. Thus, as long as the magnitude of the positive second portion B remains unchanged the second triode 22 remains conductive; however, if the magnitude of the positive slow sawtooth voltage generated by the sweep generator 11 decreases the magnitude of the positive second portion of the differentiated waveform also decreases resulting in a reduction in the positive biasing potential on the control grid 23 and a corresponding reduction in the conduction of the rst triode 21 to increase the negative potential of the cathode of the second triode 22 suiciently to render it conductive.

Coupled to the anode 29 of the second triode 22 is a cathode 34 of an electron discharge device 32 illustrated as a dual diode typically of a 6AL5 type and including an anode 35. When the second triode 22 is a nonconducting state, that is the sweep generator output is normal, both the anode 35 and the cathode 34 are substantially at the same potential, each being at the potential of the terminal 16. When the second triode 22 is rendered conductive, the potential of the anode 29 and the cathode 34 is reduced thereby initiating conduction in the diode 32 which in turn results in reducing the potential of the anode 35 due to the voltage drop across a load resistor 38.

The signal developed between the negative terminal of the sweep voltage generator 11 and ground is in a similar manner applied to the control grid 48 of a second comparator circuit 42 through a second differentiating circuit 40. The differentiating circuit 40 and comparator circuit 42 are identical in arrangement and operation to those above described. However, in this instance a clipping circuit 41 includes an electron discharge device 17 shown as a diode except poled so that the substantially larger positive portion of the differentiated waveform is shunted to ground. Accordingly, the anode 19 of the diode 17 is connected to the control grid 48 and the cathode is connected to ground. The first triode 47 therefore is biased to cut off during the negative portion of the differentiated signal.

Since the first triode 47 is biased just below its cutoff point by the negative voltage on the control grid 48 any decrease in the magnitude of this voltage, that is if this voltage becomes less negative because of a decrease in the magnitude of the negative portion of the sweep voltage, renders it conductive and decreases the voltage on the anode 49 and the cathode 36 coupled to it placing the second diode 33 in a conductive state and decreasing the Voltage of the anode 37.

The rst and second diodes 32 and 33 andV the load resistor 38 forman and gate or gating circuit 31 wherein the anodes 35 and 37 are connected in common to the load resistor 38 which in turn is connected to the terminal 16. Also connected between the anode 37 and the terminal 13 is a Voltage dividing network including the series arrange` ment of a pair of resistors 70 and 71 each of a large resistance value preferably in the order of from to l5 megohms. Theintermediate point of the two resistors 70, 71 is connected to the control grid 51 of an electron discharge device 50 shown here as a triode typically of a 12AU7 type which includes an anode 52 and a cathode 53 connected to ground. The anode 52 isl supplied with an operating potential from the terminal 16 through a coil 55 and a first set of contacts 54 of a normally closed relay 60 which includes a second set of contacts 56 connected in series with a supply voltage 61, such as 120 volts A.C. for a conventional high voltage supply generator 62 for the acceleratingelectrodes 63 of a typical cathode ray tube 64. Since the control grid 51 is maintained at a positive potential through the resistors 38 and 70, the triode 50 is in a normally conductive state. This permits current to ow from the terminal 16 to ground through l the path provided by the coil 55, the first set of contacts 54, which are held closed bythe coil 55, and the triode 5t). As long as the triode 50 remains conductive the relay remains closed; however, if the biasing potential on the control grid 51 is reduced below the cutoff value of the triode 50 the conduction ceases and the path from the terminal 1.6 to ground is broken and the contacts of the relay 6l) open.

The term an gate is used in the computer art to define a circuit which includes two input voltages and has the characteristics of a single high output voltage only when both of the input voltages are high but `if either of the input voltages are substantially low relative to the high input voltage value then the output voltage is low. As long as both the diodes 32 and 33 are in a n0nconduc tive state the voltage between the terminals 16 and 13 is divided in accordance withV the resistance values of the series resistors 38, 70, and 71, and the control, grid 51 is at a potential equivalent to the arithmetic sum of the voltage developed across the resistor 71 and the voltage between the terminal 13 and ground. If either or both of the diodes 32 and 33 are conducting, then most of the voltage drop between the terminals 16 and 13 exists across the resistor 38 because the resistance of the series combination of the resistors 70, 71 is much larger than the resistance of the series combination of either diode and its respective triode section of either comparator circuit, which latter combination is in parallel with the series arrangement of resistors 70 and 71. This places a potential on the control grid 51 sufficient to bias the diode 50 to cutoff thereby opening the relay contacts 54 and 56.

In normal operation, the cathodes 34 and 36 remain at a sufficiently positive potential to cut off the conduction in each diode and the triode 50 remains conductive to hold the relay closed. However, if the magnitude of either the positive or negative sawtooth voltages generated by the sweep generator 11 should decrease then the respective cathode of the dual diode will decrease in potential through the operation of the respective differentiat-V ing, clipping and comparator circuits coupled thereto to render the triode 50 conductive and cut off the triode switch 50 to deactivate the relay 66 and open the contacts 54 and 56. Since the contacts 56 are connected to the high voltage supply generator 62 for the acceleration anode 63 of a cathode ray tube 64 when the contacts are open the voltage supply is interrupted and the potential on the anode 63 collapses to prevent the high acceleration of the electrons emitted by the electron gun of the cathode ray tube thereby preventingrthe electrons from impinging at a high velocity on the storage surface of the cathode ray tube.

To reset the relay 60 after it has been deactivated a normally open single pole double throw lockout switch 65 is operated to shunt the protection circuits by connecting one side of the coil 55 to ground. This also connects Aa source of potential 68 such as 120 volts A.C. to a visual indicat- Y ing means 69 shown typically as a light bulb to prevent a careless operator from leaving the lockout switch engaged. f

I claim: f Y l. In combination with VafcathodeV ray tube including an accelerating electrode, a sweepV voltage generating means, having a slow sweep output voltage including a pair ofwaveforms of opposite polarity and a high voltage supply for said accelerating electrode, said protection circuit comprising: p v f a pair of diierentiating means each connected' to said slow sweep voltage generating means to receive a different one of said waveforms and to differentiate each of said waveforms to produce waveforms including in` each cycle thereof a rst `portion and a second portion of less magnitude than said first portion; Y a pair of electron discharge means each connected to Va dilerent one of said differentiating means and controllable bythe second portionyof said waveforms alegres to vary the degree of conduction when the maximum amplitude of the second portion of said waveform drops below a predetermined value;

control means coupled with each of said electron discharge means, including a pair of electron discharge devices each nonconductive when the maximum amplitude of the second portion of a different one of the differentiated waveforms is above a predetermined whereby the high Voltage supply for said cathode ray tube is disabled.

2. In combination with a cathode ray tube and a sweep voltage generating means, including a slow sweep output voltage having a pair of waveforms of opposite polarity, a protection circuit comprising:

a pair of differentiating means connected to said sweep voltage generating means each to a diierent one of said waveforms of said slow sweep voltage to produce waveforms including in each cycle thereof a irst portion and a second portion of less amplitude than said irst portion;

a pair of electron discharge means each connected to a different one of said differentiating means to be controlled by the second portion of a different one of said waveforms to vary the conduction 'thereofy when the maximum amplitude of said second portion drops below a predetermined level;

gating means including a pair of electron discharge devices each connected to a diiferent one of said electron discharge means to receive the output voltage therefrom, said gating means being adapted to dcvelop one voltage output when either of the output voltages of the electron discharge devices drops below a predetermined level and a second voltage output when both of the output voltages of said electron discharge devices are above a predetermined level;

and switch means including an electron discharge device connected to said gating means todisable the cathode ray tube when the maximum amplitude of either of said slow sweep voltage waveforms drops below a predetermined value.

3. A protection circuit for a cathode ray tube including a high voltage circuit comprising:

a slow sweep voltage generator to supply a pair of opposite polarity voltages having a sawtooth waveform;

a pair of RC networks connected between said generator means and ground to dierentiate each of said sawtooth voltages to produce a pair of waveforms having in each cycle thereof a first portion of one polarity and a second portion of another polarity of substantially less amplitude than said rst portion;

a pair of triodes each connected to a different one of said RC networks and controllable by the second portion of a diferent one of said waveforms to vary the output thereof when the maximum amplitude of the second portion of said waveform drops below a predetermined value;

gating circuit means including a pair of diodes each and switch means including an electron discharge device connected to said gating circuit means to disable the high voltage circuit of said cathode ray tube when the maximum amplitude of either of said slow sweep voltage waveforms drops below a predetermined level.

References Qited in the tile or" this patent UNITED STATES PATENTS Greenleaf et al Feb. 16, 1950 UNITED STATES PATENT OFFICE CERTIFICATE 0F CORRECTION Patent No. 3,164,746

Gordon E. Broomhall It is hereby certified that error appears in the above numbered patent reqirng correction and that the said Letters Patent should read as corrected below Column 1, line 51, for "senstive" read sensitive column 3 line 7, for "conductive" read nongonductive Column 5, line 2.5, after "to" insert differentiate Signed and sealed this 23rd day of November 1965.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents ERNEST W. SWIDER Aitcsting Officer January 5, 1965 Patent No 3 ,164 ,746 January 5, 1965 Gordon E. Broomhall It is hereby certified that error a ent requiring correction and that the sa oorreotedbelow.

ppears in the above numbered patid Letters Patent should read as Column l, line 5l, for "senstive" read sensitive Column 3, line 7, for "Conductive" read non onductive column 5, line Z5, after "to" insert differentiate Signed and sealed this 23rd day of November 1965.

(SEAL) Attest:

ERNEST W. SWIDER A t testing Officer EDWARD J. BRENNER Commissioner of Patents 

1. IN COMBINATION WITH A CATHODE RAY TUBE INCLUDING AN ACCELERATING ELECTRODE, A SWEEP VOLTAGE GENERATING MEANS, HAVING A SLOW SWEEP OUTPUT VOLTAGE INCLUDING A PAIR OF WAVEFORMS OF OPPOSITE POLARITY AND A HIGH VOLTAGE SUPPLY FOR SAID ACCELERATING ELECTRODE, SAID PROTECTION CIRCUIT COMPRISING: A PAIR OF DIFFERENTIATING MEANS EACH CONNECTED TO SAID SLOW SWEEP VOLTAGE GENERATING MEANS TO RECEIVE A DIFFERENT ONE OF SAID WAVEFORMS AND TO DIFFERENTIATE EACH OF SAID WAVEFORMS TO PRODUCE WAVEFORMS INCLUDING IN EACH CYCLE THEREOF A FIRST PORTION AND A SECOND PORTION OF LESS MAGNITUDE THAN SAID FIRST PORTION; A PAIR OF ELECTRON DISCHARGE MEANS EACH CONNECTED TO A DIFFERENT ONE OF SAID DIFFERENTIATING MEANS AND CONTROLLABLE BY THE SECOND PORTION OF SAID WAVEFORMS TO VARY THE DEGREE OF CONDUCTION WHEN THE MAXIMUM AMPLITUDE OF THE SECOND PORTION OF SAID WAVEFORM DROPS BELOW A PREDETERMINED VALUE; CONTROL MEANS COUPLED WITH EACH OF SAID ELECTRON DISCHARGE MEANS, INCLUDING A PAIR OF ELECTRON DISCHARGE DEVICES EACH NONCONDUCTIVE WHEN THE MAXIMUM AMPLITUDE OF THE SECOND PORTION OF A DIFFERENT ONE OF THE DIFFERENTIATED WAVEFORMS IS ABOVE A PREDETERMINED LEVEL AND CONDUCTIVE WHEN THE MAXIMUM AMPLITUDE OF THE SECOND PORTION OF A DIFFERENT ONE OF SAID DIFFERENTIATED WAVEFORMS IS BELOW A PREDETERMINED LEVEL; AND SWITCH MEANS INCLUDED IN SAID CONTROL MEANS, AND COUPLED WITH SAID ELECTRON DISCHARGE DEVICES TO DISCONNECT SAID HIGH VOLTAGE SUPPLY FROM SAID ACCELERATING ELECTRODE WHEN EITHER OF SAID ELECTRON DISCHARGE DEVICES IS CONDUCTING, WHEREBY THE HIGH VOLTAGE SUPPLY FOR SAID CATHODE RAY TUBE IS DISABLED. 